1. Field of the Invention
This invention relates to the recovery of the energy in gases of low heating value. It is particularly useful for the recovery of the energy in gases produced in an in-situ combustion process for the production of oil from underground carbonaceous deposits and more particularly from underground deposits of oil and oil shale.
2. Description of the Prior Art
One method for increasing the production of heavy crude oils of high viscosity from underground formations is the in-situ combustion process. In that process, air is injected at a high pressure through an injection well into the underground formation containing the heavy oil. The oil in the formation adjacent the injection well is ignited by any of several known procedures such as the procedure disclosed in U.S. Pat. No. 3,172,472 of F. M. Smith. Injection of air is continued after ignition to burn part of the oil in the formation and to increase the pressure in the formation adjacent the injection well and thereby drive oil in the formation toward a production well spaced from the injection well. A typical in-situ combustion process is described in U.S. Pat. No. 2,771,951 of Simm. The heat released by combustion of some of the oil in the formation heats the formation and oil whereby the viscosity of the oil is greatly reduced by the high temperature, cracking of the oil, and by solution in the oil of low molecular weight hydrocarbons formed by the cracking. The reduced viscosity and the pressure of the injected gases cause the oil to flow through the underground reservoir to a production well.
During in-situ combustion processes, the combustion front at which oil in the formation is burned does not move radially outward at a uniform rate in all directions. Usually the oil reservoir will vary in permeability and oil saturation from one location to another between the injection and production wells. Some of the injected air fingers through the formation toward a production well and combustion occurs at the boundaries of the fingers. There may be a breakthrough of combustion products in the nature of a flue gas long before the production of oil by the in-situ process is completed. Volatile constituents in the oil, or formed by cracking of the oil, are entrained in the injected air or flue gases and carried by them to the production well. All of these factors contribute toward variations in the composition of the gas produced from time to time during an in-situ combustion for the production of oil from a reservoir. Such variations may result in periodic increases of 100 percent or more in the heating value of the gas produced.
The fluids produced at the production well are separated into liquid petroleum products which are delivered to storage or a delivery line and gaseous products. The gaseous products customarily have been vented to the atmosphere. The gaseous products, hereinafter referred to as LHV gas, from in-situ combustion contain low concentrations of methane and C.sub.2 -C.sub.6 hydrocarbons, as well as nitrogen, carbon dioxide, sulfur compounds such as hydrogen sulfide, mercaptans and carbonyl sulfide, and in some instances a small amount of carbon monoxide and traces of oxygen. Those gaseous products constitute low heating value fuel capable of supplying a substantial part of the energy required to compress the air for injection into the subsurface formation at the injection well. The shortage of natural gas makes it important that the energy in the products from an in-situ combustion process be fully utilized. Moreover, tightening of laws relating to pollution of the atmosphere has placed stringent limitations on the amount of carbon monoxide, the sulfur compounds most frequently present in the gaseous products, and hydrocarbons other than methane that may be discharged into the atmosphere.
U.S. Pat. No. 3,113,620 of Hemminger describes a single well in-situ combustion process in which a cavity filled with rubble is formed in a subsurface oil shale deposit by means of a nuclear explosion. An in-situ combustion process in the cavity is then conducted to remove oil from the rock, aid in draining the oil into a pool in the bottom of the cavity, and force the oil up the well to the surface. The composition of the gases produced with the oil differs from the composition of gas produced in a conventional in-situ combustion process in an oil reservoir. Because of the different composition of the gas, Hemminger is able to burn the off-gas directly in a flame combustor of a gas turbine used to drive an air compressor.
Jr. U.S. Pat. No. 2,449,096 of Wheeler, Jr. describes a process for the recovery of power from gas discharged from a regenerator in a fluidized catalytic cracking process. The hot gases from the regenerator are first passed countercurrently to a nonvolatile oil in a scrubber whereby only a small amount of hydrocarbons is left in the gas. The gas with entrained hydrocarbons is passed through a catalytic oxidizer for burning the hydrocarbons and the combustion products are delivered either to a turbine for direct power recovery or to a steam generator.
U.S. Pat. No. 2,859,954 of Grey describes a power recovery system for a blast furnace in which blast furnace gas is compressed, burned, and then expanded in a turbine to provide energy for compressing air used in the blast furnace. The combustible material in the blast furnace gas is largely carbon monoxide and hydrogen. Those gases are more readily ignited and burned in dilute mixtures with inert gases than is methane. The blast furnace gas, which typically has a heating value above 90 btu/scf, is burned by Grey in flame-type combustors for release of the thermal energy.
U.S. Pat. No. 3,928,961 of Pfefferle describes the catalytic combustion of fuels to drive gas turbines. The method of Pfefferle conducts catalytic oxidations at a temperature in the range of 1700.degree. F. to 3200.degree. F., preferably in the range of 2000.degree. F. to 3000.degree. F., described by Pfefferle as the autoignition range. That temperature is high enough to initiate thermal combustion, but not high enough to cause substantial formation of nitrogen oxides. Combustion in the Pfefferle method is primarily thermal combustion. Air at least stoichiometrically equivalent to the fuel is used to complete the oxidation. When the combustion is used for gas turbine operations, the weight ratio of air to fuel is far above the stoichiometric ratio and ranges from about 30:1 to 200 or more to 1. Because the composition of the fuel is constant, the excess of oxygen in the air-fuel mixture does not cause wide variations in the temperature of the combustion products.
Because oil shale is impermeable, in-situ combustion processes for the recovery of oil from oil shale require that permeability of the shale be established before the in-situ combustion is begun. It is preferred that the permeability be established by rubblizing shale in a selected portion of the shale deposit to form an underground retort. Combustion of shale in the retort to release shale oil is preferably accomplished at low pressures to avoid leakage of gas to adjacent retorts that are being rubblized. Gases discharged from retorts for in-situ combustion processes for shale oil production are usually, therefore, at pressures too low for delivery directly into gas turbines for power recovery. Typical in-situ combustion processes for the recovery of oil from oil shale are described in U.S. Pat. No. 2,780,449 of Fisher et al. and U.S. Pat. No. 3,001,776 of Van Poollen.